I know there's a key command to do this, but cannot for the life of me remember what it is??
I'm on a Mac, btw, in case the command might be different from the pc.

Edit -> Transpose -> Transpose From...To...

Using Windows here.

This will transpose the whole song. I'm looking for the command that transposes only a section of the song.

That's exactly what you can do using that function:



Just put in the bar numbers and choose the new key and that's it

This is, as I suspected, something that just doesn't apply to the Mac version. This particular dialogue window doesn't look the same on my end nor has the same options.
So it looks like I need a MAC user to help me crack this, but thanks for the input anyway.

Or a PC to work on ..
Use a Mac for Mac stuff, but with PGMusic apps a PC has historically been better.
BiaB (for PC) is usually far ahead of the Mac version, so these options may indeed be missing from your version.
I've wondered if using a PC Emulator on a Mac would be better than using the Mac version, but haven't taken the time to find out. Of course you'd need the PC version of BiaB to test this .. so it's a weird situation.

The thing is I've actually done this once before (by mistake). I just can't remember what the bloody key combo was!

Well that has to be frustrating.
Maybe an email to Support would help. Or posting in the Mac section.

Indeed, Mac forum looks like a better bet for this one.... after the holidays